Seasonality of health events, especially
infections is well known. The seasons of high occurrence is
related to the mode of spread of the disease concerned.
Gastro-intestinal diseases, for example, are more common in summer
and rainy seasons where as respiratory infections are common in
winter season. Human births have also been reported to follow a
seasonal pattern. Cowgill, in 1966 reported a seasonal pattern in
birth of newborns in humans(1). The reason(s) for this seasonality
of births is not known and many hypotheses have been
postulated(2-8). Briefly, these can be classified as seasonality
of marriages, changes in frequency of sexual intercourse due to
differences in availability of leisure time, changes in weather
conditions and seasonality of biological chances of conception.
This study was done to document the seasonality of births in a
rural area of Haryana in north India, and to assess possible
reason(s) for the observed seasonality.

Subjects and Methods

The data for this study has been collected from
the twenty eight villages of Ballabgarh Block in District
Fardiabad, Haryana, India. These villages fall under the
Comprehensive Rural Health Services Project of All India Institute
of Medical Sciences, New Delhi. Health care to all the residents
of these villages is provided by two Primary Health Centers (PHCs).
In 1972, the estimated population of these villages was
approximately 40,000 with a birth rate of 45 per thousand, while
in 1997, the population was 70,079 and the birth rate 28.4 per
thousand.

The health care in these villages is provided
by male and female multipurpose workers (M and F MPWs), a pattern
similar to the rest of country. The data for marriages, births and
deaths are collected by male and female MPWS during their
domiciliary visits. The antenatal registration in the study area
was around 95% and immunization coverage more than 90% for the
last ten years. Initially these data were entered into a family
demographic register which had separate pages for each family.
This data of the community was computerized in 1987. At present
all births and deaths are entered into the database on a monthly
basis. Children, whose exact date of birth was not known or
children who immigrated and had no written record for the exact
date of birth were excluded from the present analysis.

In addition to routine continuous collection of
demographic information, yearly census is also conducted in the
months of May and June. This information is cross-checked by the
health assistant and the medical officer of the two PHCs for
completeness and accuracy. To adjust for different number of days
per month in a year, the monthly data was standardized by
considering a month of 30.44 days.

Results

The total number of births during these 19
years (1972-1990) was 35,720. Of these, 476 (1.3%) births were
excluded from analysis as their exact date of birth was not known.
Remaining 35,244 births were therefore included in the present
anlaysis. The maximum number of deliveries occurred in the month
of August and September. The least number of deliveries occurred
in the month of April (Fig. 1). Extent of variation around
the mean was 34.5%.

It was noticed that the marriages peaked in May
when almost two fifth of all marriages took place. The
relationship between "lagged" marriage (marriage plus
nine months) and child birth was analyzed for first born. The peak
of the two curves did not coincide. The median interval between
marriage and first birth showed a decreasing trend for marriages
taking place in the quartiles of February-April (1.80 years) to
November-January (1.62 years). The difference was however not
statistically significant (p = 0.063).

The seasonality was also analyzed in blocks of
four year periods, except for the period 1976-78 when following
forced vasectomies during 1976-78, there was a drop in the number
of births (Table I). The seasonal trend in birth remained
unchanged. Even during the period 1976-78 when the number of
births were much less, the overall pattern of seasonality remained
same.

The seasonality curves of upper and lower
castes (proxy for socio-economic status) were similar in their
months of peak and trough. However, the peak was slightly higher
for the lower castes. The effect of use of spacing method of
contraception by a couple on the seasonality of births was
assessed by comparing the curves for ever users of spacing method
and never users. The two seasonality curves were similar.

[
Fig.
1. Distribution of births of by month in the study area (1972-90)
]

Table I - Percentage
of births occurring per month during the study period (1972-1990).

1972-90

1972-75

1976-78

1979-82

1983-86

1987-90

January

7.1

7.3

8.2

6.2

6.5

7.6

February

5.7

5.9

6.2

5.4

5.9

5.4

March

4.7

4.5

5.2

4.3

4.3

5.2

April

4.4

4.2

4.6

3.9

4.3

5.1

May

5.6

6.1

5.1

5.3

5.4

6.0

June

7.4

8.1

6.2

7.9

7.5

6.9

July

10.0

9.7

8.7

10.8

10.7

10.1

August

12.5

11.9

11.7

13.4

12.8

12.7

September

12.5

11.7

13.9

12.6

13.4

11.4

October

11.7

10.8

12.3

11.9

12.1

11.6

November

10.5

11.2

10.4

10.9

10.2

9.9

December

7.7

8.6

7.5

7.6

6.8

7.9

Total
Births

35238

7596

4951

7482

7582

7627

Mean births per year

1855

1899

1651

1871

1896

1906

Fig.
2. Proposed bio-social model for seasonality of births in humans

Key Messages

There is a definite seasonality of birth
in rural India.

• The peak period for successful conception
appears to be during the winter season (November-December)

• The seasonality of births did not vary
between different socioeconomic strata.

• There appears to be a baseline biological
seasonality which can be potentiated or inhibited by external
factors like use of contraception, socio-cultural and climatic
factors.

• If interventions to decrease the birth rate, I like
increasing the use of contraception, are scheduled in the months
of November-January, they may be more effective.

Discussion

A definite seasonality in the births was
observed in the study area with peak occurring during the months
of August and September and the trough during March and April.
Seasonality of births have been reported from different parts of
the world including, Nigeria(2), Australia(3), Canada(4), United
States(5), Japan(6), Bangla-desh(7) and India(8). The amplitude of
variation (35%) was higher in our study compared to that reported
in literature. Lower values of 7% for Nigeria(2), 5% Australia(3)
and around 10-15% for Japan(6) have been reported. A study from
Bangladesh, which is similar to India in its geo-cultural
characteristic also reported a high amplitude of variation of
40%(7).

Matsuda et al.(6) reported a difference
in seasonality of the first born and subsequent born in Japan and
attributed it to seasonality of marriages. The comparison of
seasonality of marriages with first borns is valid only if the
practice of contraception does not exist among the newly wed
couples. In our area, the use of contraceptive method was less
than 5% among newly wed couples. Our study did not include
abortions and still births, estimated at 17% of all births(9).
This might influence the observed seasonality. Another factor
which could influence this relationship is the practice of Gauna
(delayed cohabitation in the event of an early marriage). This
practice was near universal in the early seventies and around 25%
in the late eighties.

The peak in births in the months of August and
September implied that there is increased conception during
December and January. It has been postulated that high summer
temperatures reduce conception directly (affecting ovulation or
spermatogenesis or fetal loss) or indirectly (reduced sexual
intercourse due to physical discomfort).(2,3,5,7,8). Most of the
houses, in the study area consisted of one or two rooms. The
average number of family members ranged between four and five.
There is a lack of privacy in these houses and winter season may
actually reduce the opportunity for sexual activity since all
family members may sleep inside.

Availability of leisure time due to slackness
in agricultural work may result in increased sexual activity.
During the peak agricultural work especially during the harvest
(April to May), most of the villagers work till late night and
often sleep in the field resulting in decreased sexual activity.
Over the years there has been a change in the agricultural
practice in the study area. Though, wheat continues to be the
major crop, cultivation of other cash crops like sunflower, maize,
sugar cane is gradually increasing. However, there has been no
change in the seasonality of births during these years.

In United States, a seasonality similar to this
study (peak in August and September) was explained by festival
season of Christmas and New Year(1). In Malaysia, the seasonality
of births in sixties varied among the indigenous Malayan (peak in
October) and the Chinese (peak in January) immigrants(10). Authors
attributed this difference to religious obser-vances/holidays. The
amplitude of the variation was also lesser among the Malayans
probably because they had changed their traditional practices(10).

The availability of contraception may influence
the seasonality of births as couples can now choose the time of
conception. The couple protection rate (CPR) in the study area
increased from less than five per cent in 1972 to around 35% in
1990. A major proportion to CPR is contributed by females
undergoing tubal ligation after completing their families.

Our study area is a predominantly agri-cultural
community with even those working in urban areas returning to the
villages during harvesting and sowing seasons. The staple diet in
these areas is wheat which is harvested in April and May. The
seasonality in the availability of food may result in seasonality
of malnutrition in the population(11-13). There may, however, be a
latent period of some months between the change in nutritional
status and its influence on reproductive biology. Many studies
have reported a shift in seasonality over the years. This has been
reported from Japan(6), United States(1), Malaysia(10), and
Canada(4). The important reasons suggested for this are increase
in use of birth control methods in Canada(4), industrialization
and americani-zation of way of life in Japan(6), changes in way of
life and decreased religious practice among Malayans(10).

Based on the review of literature, our study
findings and our experience of working in the community, we
propose a model which tries to explain the seasonality of births
in different areas of the world (Fig. 2). We believe that
there is a baseline seasonality of births. This baseline
seasonality is subjected to push and pull from two groups of
factors: potentiating (push) factors and inhibiting (pull)
factors. The potentiating factors push the curve away from
baseline. Thus, they could be the factors which accentuate the
peak or which depress the trough further. This includes
holidays/festivals, condu-cive weather, agricultural cycle,
seasonality of marriage, use of contraception. The inhibiting
factors are the factors which mask the biological seasonality of
conception. They, thus, pull the curve towards the baseline. These
include air conditioning of homes, contraception, religious
proscription, food availability; all features of socio-economic
development. The overall seasonality depends upon the combined
influ-ence of potentiating factors and inhibiting factors on the
baseline seasonality. In less developed communities, the
potentiating (push) factors may be more important and as the
development progresses the inhibiting (pull) factors may assume
greater importance. A very high seasonality seen in this study
could be because of the presence of potentiating factors like
temperature differential, agricultural leisure time. But, more
importantly there is an almost complete lack of any inhibiting
factors.

We suggest further studies to address issues
related to seasonality of births. Firstly, studies should be done
to find out factors causing baseline seasonality and quantify it.
Our hunch is that it will be around 15%. Future studies should
look into the questions of seasonality of ovulation, survival of
ovum, sperm, zygote, etc. Secondly, there is a need to test
the validity of the proposed model by comparing studies from
different geographical areas of the world on the factors listed
above. It may be possible to have a mathematical model based on
the bio-social model suggested above. We hope all subsequent
studies will provide data on the factors listed in our model so
that a comprehensive evaluation of the proposed model can be
carried out.

Contributors:
SK conceived the idea of the study and took part in supervision of
data collection, and helped in analysis and drafting. He is the
guarantor of the paper. GK was responsible for handling the data
from collection, computer entry and analysis. KA supervised data
collection, conceived the pattern of analysis and wrote the draft.
SK was involved in data analysis and revising the draft.

11. Anand K., Sundaram KR, Lobo J, Kapoor SK.
Are diarrheal incidence and malnutrition related in under five
children - A longitudinal study in an area of poor sanitary
conditions in India. Indian Pediatr 1994; 31: 943-948.